The amount of current that can be supplied by the transformer depends upon the VA [ Volt-Ampere] rating of the transformer.
Do you have any more details for the transformer.? any labels on the transformer.?

If not, measure the area of the centre section of the transformer and also the diameter of the wire on the 9v,0v,9v windings, let us know what you measure.
A photo of the transformer would help.

Well the usual configuration is that from the 0V terminal there is 9volts to either of the other two. Research "full wave rectifier" or center tapped transformer and you should then see how it is arranged. Without a picture or more info of your transformer it is difficult to be more specific. If it has 3 leads, sometimes two are the same colour, with the third being a different colour, this is the 0V lead. Also, if you do not use the middle wire, you will have 18 volts between the other two. Think of the secondary as being an 18volt winding with a connection at the middle part of the winding. Then you can configure the connections for your particular application.

Be aware that some transformers ratings are for a particular load, and although marked 9volts, may well measure higher, peak voltage is approx 1.414x R.M.S (and depending on whether your meter reads true RMS or average) and will only be correct at a a specific load, also many transformers secondary voltage rating is the assumed D.C voltage from a rectified and smoothed output at the designed load. That is altogether another subject that involves the conduction period and diode specs and the correct value of reservior smoothing capacitor.